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#1
posted to rec.boats.electronics
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A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Paul Hovnanian P.E. wrote:
Think of the shore power ground as a connection to the rest of the world. A world with structures and ground rods that may be in contact with sea water, but have no provisions for galvanic protection themselves. If you have zincs on your boat, there will be a current (and your zincs will dissolve). If you break that circuit, you will see the voltage of that galvanic cell (the one between your zincs and the rest of the world). Well, that's only half of the situation. Your prop and any underwater metals will also form galvanic cells with the rest of the world, of different polarity and potential difference. There is a burden here of demonstrating that on average these do not cancel, and a greater burden of demonstrating that their net effect is a 1.2 volt cell. But the 1.2 volts measured was NOT an open-circuit voltage measurement, but one across a functioning GI. The discussion has gone open loop. Here is a recap. 1. An assertion was made that 0 VDC across a GI means the diode(s) are shorted. 2. I suggested that 0 VDC also meant the absence of galvanic currents through the shore power ground wire; a good thing. 3. A counter-assertion was made that there is always 1.2 VDC across a properly functioning (NOT open-circuit) GI due to the boat's zinc. 4. I suggested there was no path for the boat's zinc/bronze galvanic current to pass through the shore power grounding wire and some other explanation was called for. 5. An assertion was made that the galvanic couple in 4 made the boat "live" and that led to the measured 1.2 VDC across the GI. 6. I responded that the assertion failed to identify the current path by which this occurred. Further, I observed that a measured forward voltage of 0.6 volts per diode was equivalent to a forward current on the order of 100 mA, and that was far in excess of the currents found in typical yacht-based galvanic couples. 7. A contrary assertion was made that at 0,6 volts, the forward current in a diode is on the order of only microamps. And so here we are. I don't know your basis for that assertion, Paul. Disregarding what has come to be the normal protocol for some in the group, I actually measured a 1N1190A (I use them in the GI's I build) a few moments ago. The forward current at 675 millivolts is 100 milliamperes. Using a Keithley electrometer, I measured a forward current of about 10 microamps at a voltage of 300 mV, consistent with the 675 mV/100 mA measurement. Even the 1N914 signal diodes pass almost one mA (about 700 microamps per the datasheet) for a forward voltage drop of 600 mV. Here is where the discussion stands: I. An observed anomalous current of ~100 mA DC is measured across a GI. (Actually the measurement was 0.6 volts DC and there is disagreement over every aspect of that measurement.) II. The current path (of 100 mA) between the boat's zinc/bronze couple and the shore power ground has not been identified although much hand-waving has transpired. III. There is seemingly irreconcilable disagreement about metrology, Ohm's law, diode VI characteristics (e.g., the switch analogy), and the properties of galvanic currents. Your patience with me is appreciated, but there are other callings. Chuck PS: Paul, I inadvertently seem to have sent an earlier draft of this directly to you rather than to the group. No idea how that happened, but my apologies. ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#2
posted to rec.boats.electronics
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A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
chuck wrote in news:1153748524_106745
@sp6iad.superfeed.net: Using a Keithley electrometer, I measured a forward current of about 10 microamps at a voltage of 300 mV, consistent with the 675 mV/100 mA measurement. At this level, a lot of what the Keithley is measuring is the diode's own generated voltage. Hook the Keithley to a silicon diode sitting on the bench, not attached to anything. Depending on how hot it is (and how close to any radioactivity it is), there's always a junction voltage from the thermionic emission of the junction, itself. Got a hot radioactive source around? Keep the Keithley across the diode and move the diode up against any beta or gamma sources and watch it fly... (c; (Depleted uranium bullets are great for this experiment. They're free in Afghanistan and Iraq...) Which Keithley is it? I used to repair and cal them at the Metrology Laboratory of the Quality Assurance Office, Charleston Naval Shipyard (Code 132)...may she rest in peace. |
#3
posted to rec.boats.electronics
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A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
Larry wrote:
At this level, a lot of what the Keithley is measuring is the diode's own generated voltage. Hook the Keithley to a silicon diode sitting on the bench, not attached to anything. Depending on how hot it is (and how close to any radioactivity it is), there's always a junction voltage from the thermionic emission of the junction, itself. Indeed! Actually the junction voltage was ~50 millivolts so I "tuned it out" as they say. Got a hot radioactive source around? Keep the Keithley across the diode and move the diode up against any beta or gamma sources and watch it fly... (c; (Depleted uranium bullets are great for this experiment. They're free in Afghanistan and Iraq...) I could try a smoke detector or Coleman lantern mantle with thorium, I guess. I'll pass on the DU though. Which Keithley is it? I used to repair and cal them at the Metrology Laboratory of the Quality Assurance Office, Charleston Naval Shipyard (Code 132)...may she rest in peace. It's a 610B with electrometer tubes. Works amazingly well for its age. It does have a personality as you know. Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#4
posted to rec.boats.electronics
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A QUICK CHECK OF YOUR GALVANIC ISOLATOR.
chuck wrote:
Paul Hovnanian P.E. wrote: Think of the shore power ground as a connection to the rest of the world. A world with structures and ground rods that may be in contact with sea water, but have no provisions for galvanic protection themselves. If you have zincs on your boat, there will be a current (and your zincs will dissolve). If you break that circuit, you will see the voltage of that galvanic cell (the one between your zincs and the rest of the world). Well, that's only half of the situation. Your prop and any underwater metals will also form galvanic cells with the rest of the world, of different polarity and potential difference. There is a burden here of demonstrating that on average these do not cancel, and a greater burden of demonstrating that their net effect is a 1.2 volt cell. But the 1.2 volts measured was NOT an open-circuit voltage measurement, but one across a functioning GI. The discussion has gone open loop. Here is a recap. 1. An assertion was made that 0 VDC across a GI means the diode(s) are shorted. 2. I suggested that 0 VDC also meant the absence of galvanic currents through the shore power ground wire; a good thing. Either 1 or 2 could be true. 3. A counter-assertion was made that there is always 1.2 VDC across a properly functioning (NOT open-circuit) GI due to the boat's zinc. No. Less than 1.2V for this case. The potential between Zn and Cu in a 'standard cell' is about 1.1V. It is likely to be less than that in sea water. Cu (copper) is one of the most electrically positive metals you are likely to find in common use. So actual potentials are likely to be much lower. 4. I suggested there was no path for the boat's zinc/bronze galvanic current to pass through the shore power grounding wire and some other explanation was called for. - your boat - - the rest of the world - +-------+------------shore pwr gnd-------+----------+-------+--------+ | | (w/o GI) | | | | Zn Bronze more bronze steel aluminum copper -------------------------------- sea water ---------------------------- There are parallel paths through your zincs, your own prop (bronze) and what might be tons of metal in the outside world. While your zincs will protect your prop, they will also 'protect' the rest of the world. Since your zincs dissolve to provide this protection, they will just dissolve faster when connected to more metal. End result, your zincs get eaten up in days. 5. An assertion was made that the galvanic couple in 4 made the boat "live" and that led to the measured 1.2 VDC across the GI. The galvanic couple might generate a few tenths of a volt between 'the world' and your boat ground. If you see something near 1.2 volts, there is something else going on. The GI diodes will maintain a 1.2 volt drop for current levels from 10s of milliamps to many amps, so if you see 1.2V, suspect trouble. 6. I responded that the assertion failed to identify the current path by which this occurred. Further, I observed that a measured forward voltage of 0.6 volts per diode was equivalent to a forward current on the order of 100 mA, and that was far in excess of the currents found in typical yacht-based galvanic couples. 0.6 V per diode is close to the knee in its I-V characteristic. Depending on the exact diode, that might mean that it is conducting 10 mA. Or maybe 1 A. The various opens and shorts that might combine to put this kind of current (plus amateur wiring jobs) are too numerous to list here. There shouldn't be any galvanic reactions that will exceed a double diode (1.2V) drop. 7. A contrary assertion was made that at 0,6 volts, the forward current in a diode is on the order of only microamps. No. I said _below_ 0.6 volts. And so here we are. I don't know your basis for that assertion, Paul. Disregarding what has come to be the normal protocol for some in the group, I actually measured a 1N1190A (I use them in the GI's I build) a few moments ago. The forward current at 675 millivolts is 100 milliamperes. Using a Keithley electrometer, I measured a forward current of about 10 microamps at a voltage of 300 mV, consistent with the 675 mV/100 mA measurement. Even the 1N914 signal diodes pass almost one mA (about 700 microamps per the datasheet) for a forward voltage drop of 600 mV. And 100 uA at 500 mV and 12 uA at 400 mV. Its an exponential function and current drops off very rapidly for forward voltages below 600 mV. Here is where the discussion stands: I. An observed anomalous current of ~100 mA DC is measured across a GI. (Actually the measurement was 0.6 volts DC and there is disagreement over every aspect of that measurement.) Did you actually measure that current? Given your above measurements, a GI (with good diodes) should not conduct 100 mA with a 0.6 volt drop (I'm assuming a double diode drop in each direction). II. The current path (of 100 mA) between the boat's zinc/bronze couple and the shore power ground has not been identified although much hand-waving has transpired. If the above measurements are valid (both I and V), I'd assume that one of the GI diodes is bad (shorted). 100 mA at 0.6 V forward is near the VI curve I'd expect for a single diode. III. There is seemingly irreconcilable disagreement about metrology, Ohm's law, diode VI characteristics (e.g., the switch analogy), and the properties of galvanic currents. Your patience with me is appreciated, but there are other callings. Chuck PS: Paul, I inadvertently seem to have sent an earlier draft of this directly to you rather than to the group. No idea how that happened, but my apologies. That's OK. I'll respond here, so others may comment. ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- -- Paul Hovnanian ------------------------------------------------------------------ Marching to a different kettle of fish. |
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